At first sight, Whistle appears to be a sweet and seemingly harmless puppy, but somewhat similar to Flippy, whenever he hears a whistling sound, (such as a whistling tea kettle, a bird whistling, or someone blowing a whistle) he grows savage and blood-thirsty (like Fliqpy), killing and/or maiming the closest person. After some time (usually after killing someone), he turns back into a regular puppy. In Doggone It, there are dozens of dogs just like Whistle discovered in the back of Lifty and Shifty's van, all of who go insane upon hearing a whistle. Whistle also served as The Mole's seeing-eye dog in A Sight for Sore Eyes, though he had yellow fur and was not aggressive.

A dog similar to Whistle or a different version of Whistle appeared in the HTF Break short Chore Loser, only it was blue and attacked people without hearing a whistle. The Cursed Idol was in this episode, however, making it possible that it was the cause of his aggression. Then again, the Cursed Idol has the appearance of a fake one though, so it could be a different type of dog, but attacked only when it was petted.

Whistle is light gray in color and has a blue mark around his eye. All of the countless other dogs Lifty and Shifty stole in Doggone It were either the same color he was or grayish-turquoise or purple, and each had the notable eye ring around a different eye each.

Cub's Death

The concept behind Whistle's killing method is to kill something, then he returns to his friendlier mentality, and there has been confusion if he has killed Cub in Doggone It. It is actually fair to assume Whistle hasn't killed Cub because Whistle was still acting savage even after Cub was attacked and he started attacking Pop. Despite this statement, there are two points to make that go against this statement: the first is that Cub could've bled out while Pop was being attacked, as he was very badly injured; the other being that Whistle is supposed to return to normal after killing someone but not necessarily, as it is common for an animal to not be 100% consistent in their behavior. However, it can be argued to defend this point as Whistle only ever went back to his friendly state when no one saw him kill anything, so he may have been caught in the act by Pop, and that spurred him on to attack Pop.